Whenever a wireline telephone user places an emergency access call, such as by dialing 911, automatic number identification information (ANI) signals embedded in the call are employed by a database search mechanism to automatically provide the location of the calling subscriber circuit to a Public Safety Answering Point (PSAP). However, when a 911 type of call is placed from a wireless communication device, such as a cellular telephone, phone location information cannot be automatically provided to the PSAP, since the calling cellular phone is a mobile rather than a fixed piece of equipment. Because of this shortcoming, the Federal Communications Commission has mandated that steps be taken by the communications industry to make cellular phones geolocation capable to within a prescribed accuracy (one hundred twenty-five meters). In addition to meeting this emergency services requirement, automatically providing geolocation information also facilitates a number of ancillary functions, such as location-based billing, locating a stolen or misplaced phone, fraud reduction, and tracking and location capability for dispatch, delivery and sales personnel.
Current proposals to satisfy the above objectives include relatively complex and costly trilateration schemes that are based upon differences in time or angles of arrival of a timing fix or reference signal transmitted by the cellular phone and received at three or more geographically spaced apart receivers. Other proposals include the use of a phased array antenna and a pair of receiver stations to determine angle of arrival and difference in time of arrival for triangulation purposes. Another proposal is to require that all currently employed cell phones be retrofitted with additional signal processing circuitry, such as that including a global positioning system (GPS) processor-based receiver, through which the cell phone determines its position and then transmits that information as part of the 911 call.